From "AN HISTORIC DESCRIPTION OF AWARENESS HOLDERS OF THE GREAT SECRET MANTRA WHO ARE RESPLENDENT IN WHITE CLOTHES AND LONG HAIR",
a brief oral commentary by Kyabje Kunzang Dorje Rinpoche: http://saraswatibhawan.org/an-historic-description-of-awareness-holders-of-the-great-secret-mantra-who-are-resplendent-in-white-clothes-and-long-hair/
There are two types of ngakpas – those of family lineage (rigs rgyud) and those of Dharma lineage (chos rgyud). Ngakpa family lineages are passed from father ngakpa to their sons from generation to generation. At present, these are family lineage holders such as the great lamas of the Nyingma tradition, Minling Trichen Rinpoche and Sakya Trinzin Rinpoche, the throne holder of the Dharma Potrang lineage.
There are Dharma lineage nagakpas in both the Nyingma and Sarma traditions. Since one may enter the Tantric mandala by receiving empowerment, scriptural authorization and practical instructions from a qualified Lama, it is not necessary to be born into a ngakpa family lineage. Once one has properly received these transmissions, one must authentically enter into the sadhana practices of approach, accomplishment and activity.
Ngakpas such as these allow their hair too remain long and uncut. They dress in simple, white clothes. Their minds reside in the unfabricated, natural state. These are the three aspects of the ngakpa’s non-contrivance (ma bcos rnam gsum gyi sngags pa).
Further, in colloquial language, there is a custom of referring to ngakpas as ‘white,’ ‘black’ and ‘multi-colored.’ Those who rely on alms and essence extraction as food, mystic heat and a single piece of cotton for clothing, and fully integrate their lives with sadhana practice are called ‘white ngakpas.’ Further, those who engage in sadhana practice in solitary retreat for only three months out of the year and perform rituals for lay people are called ‘multi-colored ngakpas.’ Similarly, ngakpas holding the family or Dharma lineages that spend less than seven days in retreat but perform village rites, are known as ‘black village ngakpas.’ These are well known designations in colloquial language.
These days in Tibet, there are only three ngakpa gomdes that are very well known. In the region of Amdo, there are the Rekong (reb kong) ngakpas who generally wear long, matted hair, a multi-colored shawl worm across the shoulder and red clothing. The Vajra Masters of this gompa are similarly attired, although they usually wear a white skirt.
Similarly, at Chakri Phurdrak (chags ri’I phur brag) gomde, a place where there is a spontaneously appearing letter ‘A’ on a rock, the renowned ngakpas who serve the government wear clothing similar to the general ngakpa’s attire described above.
On the border of U and Tsang, in Shang Zabphulung (shangs zab phu lung), there is a community of ngakpas known as ‘Zabphu’ zab phu’ ngakpas. These yogis wear uncut hair, multi-colored shawls and white skirts. There, once one has completed the general practices, the accumulations and purifications of the preliminary practices and has received empowerment, scriptural authorization and practical instructions for Lama Gongdu, one is allowed to wear the white clothing. When I was 27 or 28 years old, I lived at this ngakpa gomde for several years.
Generally speaking, in Tibet, there are many ngakpa gomdes, but one cannot possibly explain in detail the descriptions, histories and so forth of each and every one of them.